Well, I finally finished up a patch that adds postquote support to
SFTP, allowing users to use commands like OpenSSH's sftp interactive
commands during an SFTP session. This patch is against the latest
version in CVS.

The commands supported are chown, chmod, chgrp, ln/symlink, mkdir,
rm, and rmdir. Syntax is the same as with the OpenSSH sftp command.
If there's a syntax error, the session will end with a
CURLE_FTP_QUOTE_ERROR.

I borrowed some parsing code from OpenSSH, which uses the BSD
license. I hope that's OK...

I also only tested the rm command, since that's the only one I really
need, but the others ought to work...